Nanotechnology is defined by the Institute of Nanotechnology in the UK as “science and technology where dimensions and tolerances in the range of 0.1 nanometers (nm) to 100 nm play a critical role”.
Research in the field of nanotechnology is a rapidly expanding industry and includes the production of nanostructures—structures which have at least one dimension at least approximately on the scale of 0.1 to 100 nm (referred to as the “nano-scale”). Two examples of nanostructures are nanowires and nanotubes, which each have a nano-scale cross section.
Methods of forming nanostructures include methods based on the miniaturisation of technology developed initially at a scale greater than the nano-scale, and new methods for the formation of nanostructures from molecular elements.
Current methods of forming nanowires are usually electrochemically based and involve the use of porous materials such as alumina. The pores of such materials are, for example, channels having the desired dimensions for the nanowires to be formed. Ions of the metal from which the nanowires are to be formed are contained in an electrolyte and are drawn into the channels by applying a voltage across two electrodes being positioned at either end of the channels. Once the nanowires are formed, extracting the nanowires from the porous material commonly involves removal of the porous material.
The nanowires formed in this process are often collected as a powder and the collection and manipulation of individual nanowires is a relatively time consuming, skillful and cost inefficient process. To date there is no automated process for the reliable manipulation of individual nanowires. This makes the use in industry of the nanowires produced by this process impractical. Additionally, the pores of the porous material are often machined to meet desired dimensions of the nanowires to be formed. This proves to be difficult if the dimensions of the pores are required to meet strict tolerances.
Scientific paper “Synthesis and magnetic behavior of an array of nickel-filled carbon nanotubes”, Applied Physics Letters 81, 4592 (2002) describes a method of forming nickel-filled carbon nanotubes. In this method hollow carbon nanotubes are first formed within pores of an alumina membrane. For this, acetylene gas (C2H2) at a relatively high temperature of 700° C. is used. Nickel (Ni) is then deposited inside the hollow nanotubes by an electrochemical method similar to that described earlier. Once the nanowires are formed, the alumina is removed and, in this method, the nickel-filled nanotubes are obtained as an ordered array in which the nanowires are aligned with each other. Despite this order and alignment, any manipulation of individual nanotubes is difficult. Additionally, the relatively high temperature of the method prevents use of reagents which are unstable at such temperatures. The use of acetylene gas is relatively hazardous as the gas is flammable, especially at relatively high temperatures.
Carbon nanotubes are commonly formed on a substrate having a ferromagnetic catalytic layer of for example, nickel. In general, a carbon vapour plasma is formed by the decomposition of a gas, for example acetylene, due to heating and/or an application of an electric field. The carbon of the plasma reacts with the catalytic layer to form nanotubes which form vertically from the substrate and are approximately aligned with each other. Often the catalytic layer forms individual particles upon heating, each individual particle leading to the formation of one nanotube. Aligned nanotubes only usually form in the presence of an externally applied electric field.
The scientific paper “Uniform patterned growth of carbon nanotubes without surface carbon”, K. B. K. Teo, M. Chhowalla, G. A. J. Amaratunga, W. I. Milne, D. G. Hasko, G. Pirio, P. Legagneux. F. Wyczisk, and D. Pribat, Applied Physics Letters 79, 1534 (2001) describes a method of growing vertically aligned nanotubes at precise locations on a substrate. A nickel catalytic film on a substrate forms nanoparticles of the nickel at 700° C. Ammonia and acetylene gases are introduced at this temperature and, using a plasma-enhanced chemical-vapour deposition (PECVD) technique, nanotubes are grown from the nickel nanoparticles, vertically to the substrate. An electric field induced by the plasma causes the nanotubes to be aligned.
The scientific paper “Large-area synthesis of carbon nanofibres at room temperature”, B. O. Boskovic, V. Stolojan, R. U. A. Khan, S. Haq, and S. R. P. Silva, Nature Materials, 165 (2002) describes a method of forming carbon nanofibres at room temperature, 100° C. and 250° C. A well-formed nanotube may be considered to comprise a hollow tube having walls formed of curved sheets formed of, e.g. graphite. Each end of the hollow tube is capped with a fullerene hemisphere, typically formed of carbon. A nanofibre may be considered as a nanotube in which the sheet and fullerene structures comprise defects. In this method the nanofibres are formed from nickel particles on a substrate using methane gas as the source of carbon. A radio-frequency voltage applied across the volume of methane creates a hydrocarbon plasma which provides the carbon required for nanofibre formation at the surface of the nickel particles. A nickel particle remains at a tip of each growing nanofibre and, although this method may be performed at room temperature, the radio-frequency PECVD causes superheating of this tip to a temperature of approximately 450-1250° C. to provide a required energy for the reaction. The nanofibres formed by this method are not aligned but have what is referred to as a ‘spaghetti morphology’.
In both these methods of the prior art for the formation of nanotubes using a PECVD technique, a relatively high temperature for the formation reaction to occur is required, irrespective of a surrounding temperature. Such relatively high temperatures prevent the use of materials which are unstable at such temperatures. Additionally the use of acetylene or methane gas, which is commonly used in similar nanotube forming reactions, is relatively hazardous as the gas is flammable, especially at relatively high temperatures.
It is an object of the present invention to provide improvements to methods of forming nanowires and nanotubes, particularly at relatively low temperatures.